1. Field of The Invention
This invention relates to a system for monitoring the transmission capability of power transmission lines. More particularly the invention is directed to an apparatus and method for determining the current carrying capacity of power lines which is limited by the allowable sag of the line between transmission towers. To control the load on a power line it is necessary to know the condition of the line in terms of the sags of the different spans. The present invention may incorporate the method and apparatus set forth in applicants U.S. Pat. No. 5,235,861 entitled Power Transmission Line Monitoring System the disclosure of which is herewith incorporated by reference.
2. Description of the Prior Art
Certain problems associated with overhead power transmission lines regarding sag and clearance between the line and the nearest point on earth, are set forth in applicant's above cited prior patent. This patent also outlines prior art solutions to this problem and cites certain prior patents. The system for measuring power line sag between spans described in the 861 patent employs a novel tension monitoring system in which the line tension of the power line is measured and appropriate calculations are made as outlined in the patent in order to determine sag by which the person controlling the power line can appropriately adjust the load.
The calculations for converting tension to sag or power line capacity, rely upon a concept known in the power transmission art as Ruling Span which is a calculated weighted average of span lengths between deadend power line towers. Ruling span is used to design transmission lines and then subsequently used in monitoring sag of the lines. The Ruling Span between the deadend structures is defined by the following formula. ##EQU1## in which l.sub.1 . . . l.sub.n are the span lengths in the ruling span section between deadends. There are several assumptions underlying this concept. The principle ones are the following:
(1) The span lengths between deadends do not vary significantly. PA1 (2) All structures are rigid and the line section is straight, without angle structures. PA1 (3) Insulator strings are long compared to the difference of the thermal expansion between the spans. PA1 (4) Elastic, thermal elongation and creed properties of the conductors are accurately known based on manufacturer's data. PA1 (5) Conditions of the line during sagging are known accurately. PA1 (6) Subsequent events affecting creep of conductors (conductor temperature and tension) can be accurately estimated.
A more detailed description of the Ruling Span concept is described in an article Ruling Span Concept is Reexamined for Sagging Techniques by J. Brian White found in Transmission and Distribution International, September 1992 pages 18 and 19.
If the Ruling Span length and the above properties (4..6) are accurately known, the sag of the conductor in any of the spans of the ruling span section between deadends can be calculated with a high degree of accuracy using commercially available sag/tension programs.
Actual tension measurements on transmission lines have indicated that the above assumptions are often not very reliable. The accuracy becomes quite important when transmission lines are operated in conditions near their specified clearance limits, which are the limiting factors for maximum currents of the transmission line.
Methods exist for determing the sagging accuracy by observations.
The traditional method has been based on daytime observations of either sag of the line using optical devices, such as transits or by observing the propagation velocity of transverse vibration of the conductor (bounce method). Such observations are, of necessity, made during daytime. Sag measurements using such techniques are subject to two main error sources: (1) If the line carries a current, the temperature of the conductor cannot be accurately estimated and (2) even if the line is de-energized, the solar radiation on the conductor can cause up to 15.degree. C. temperature rise compared to ambient.
In the past, the accuracies of the sag measurements by such observations were considered sufficient by electric utilities, because very few lines were ever operated near or at their maximum sag conditions. The recent emphasis of operating lines closer to their thermal limits has brought about the need to increase the accuracy of the calculations.